1. Field of the Invention
The present invention relates to a pacemaker of the type having stimulation threshold measuring means for measuring the stimulation threshold voltage value.
2. Description of the Prior Art
It is advantageous to use stimulation pulses of short pulse widths and corresponding high amplitudes which gives a low noise to signal ratio, since with such stimulation pulses the pacemaker current consumption is lower, which results in an increased lifetime of the pacemaker battery, and the polarization is reduced, which e.g. facilitates the detection of evoked response. Thus, it is preferable to use short stimulation pulses as long as the associated stimulation amplitude is below the battery voltage, i.e. as long as voltage doublers are not needed, of U.S. Pat. No. 5,391,191.
Since numerous factors have a changing influence on the stimulation threshold the stimulation must be performed with a safety margin. A 100% safety margin was previously manually set, which means that the energy delivered for pacing was 100% higher than the minimum amount of energy necessary for exciting the heart at the time of control. This known technique is illustrated in FIG. 1a for a constant pulse width.
In automatically threshold searching pacemakers of today stimulation pulses are normally delivered with an amplitude equal to the stimulation threshold value Uthres plus a safety margin m, thus
Ustim=Uthres+mxe2x80x83xe2x80x83(1)
If a constant safety margin of e.g. 0.3V is used (which is illustrated in FIG. 1b), independent of the threshold value Uthres, this safety margin is relatively smaller for high amplitude pulses than for pulses of lower amplitudes. This means that when using short stimulation pulses with corresponding high amplitudes, loss of capture occurs frequently and results in several back-up pulses and triggered threshold searches, which disturbs the proper function of the pacemaker.
U.S. Pat. No. 4,979,507 deals with the problem of controlling the pacemaker pulse energy in operation most efficiently to prolong battery life by matching of individual patient pacemaker-implant electrode interface to follow dynamic changes occurring in use. It is then shown that the stimulation energy consumption is lowest for pulse widths corresponding to chronaxie of the strength-duration characteristic. The stimulation threshold at the time of chronaxie is determined by a two point measurement and is continuously checked and the stimulation is prescribed to be performed with an added safety margin in the range of 5-30%. This safety margin can be added by elevating either the voltage amplitude or the pulse width, the latter being preferred according to this patent. Thus a conventional safety margin is added to the threshold value determined at the chronaxie to achieve the lowest energy.
Also U.S. Pat. No. 5,447,525 deals with the problem of minimizing current drain while maintaining a desired capture safety margin. Either periodically or as a result of a loss of capture, the pacemaker then calculates for each possible width a threshold value based upon 10 the measured rheobase and derived chronaxie. Each threshold value is multiplied by a pre-determined safety margin. As pacing stimulation signal is then the voltage-pulse width pair chosen having the lowest current drain and satisfying the desired safety margin.
An object of the present invention is to reduce the number of losses of capture by a new and reliable technique which avoids the disadvantages of known manners of adding a safety margin.
The above object is achieved in accordance with the principles of the present invention in a pacemaker having a stimulation threshold measuring arrangement for measuring the stimulation threshold value, a unit for generating a safety margin, and a pulse generator for deliverying stimulation pulses of variable amplitudes and duration under the control of a control unit which sets the amplitudes dependent on the measured threshold value and the safety margin, wherein the unit for generating a safety margin automatically changes the safety margin in accordance with a predetermined relationship between the safety margin and the measured threshold value, this relationship being such that the safety margin is progressively increased when the measured threshold value increases.
Thus, with a pacemaker according to the invention, stimulation pulses are delivered, which are progressively increasing with increasing values of the measured threshold value for the actual pulse duration, i.e. a dynamically adjustable safety margin is provided, which is based on threshold measurements and is progressively increasing with increasing threshold value.
In a embodiment of the pacemaker according to the invention said safety margin generating means is adapted to increase and decrease the magnitudes of the safety margin m in predetermined steps, when the threshold value is increased above and decreased below predetermined threshold limit values. By limiting the safety margin to only a limited number of steps the electronics of the safety margin generating means can be simplified, yet a considerable improvement of the proper functioning of the pacemaker is obtained vis-a-vis use of a constant safety margin. Even if the safety margin is varied between only two prescribed level the number of losses of capture is considerably reduced compared to a situation of a constant safety margin.